In some forms of rotary percussive drill, hammer blows are imparted to a drill bit or other tool either directly or indirectly by a ram reciprocated by a hollow piston through an air cushion formed between the ram and the piston in which the ram is mounted. When the drill is to operate without a hammer action, it has been proposed to allow the ram to move further forward so that a port in the piston is uncovered and the air cushion between the ram and the piston is vented so that although the piston continues to reciprocate the ram does not. Another proposal has been to provide a mechanical latching arrangement of some kind which frictionally engages the ram in its most forward position.
However, it is found that the continuing reciprocation of the piston can lead to the ram continuing to reciprocate, especially if the speed of the drill is infinitely variable so that the forces on the ram are variable and the speed of the drill can coincide with the natural resonant frequency of the reciprocating ram. Such reciprocation is disadvantageous because of the noise and vibration which it produces.